Added first draft of "quickstart" instructions to documentation.
Don't clone git, download release tarball.
Polishing of quickstart guide.  Minor fixes, more information, formatting.
Small tweaks.  More introductory text.
Micro-optimization.
Fixed directories in instructions.
Fixed commands regard downloading and preparing shogun-data.
Merge pull request #1867 from tklein23/develop

Added first draft of "quickstart" instructions to documentation.